1. Field of the Invention
The present invention relates to a bipolar plate for a direct liquid feed fuel cell stack.
2. Description of the Background
A direct liquid feed fuel cell generates electricity by electrochemical reactions between an organic chemical compound such as methanol or ethanol and an oxidant such as oxygen. A direct liquid feed fuel cell has high energy density and high power density. The direct liquid feed fuel cell uses methanol, etc. directly as fuel, so external peripheral devices such as a fuel reformer are not required and the fuel can be easily stored and supplied.
Referring to FIG. 1, a single cell of a direct liquid feed fuel cell has a membrane electrode assembly (MEA) structure in which an electrolyte membrane 1 is interposed between an anode 2 and a cathode 3. The anode 2 and the cathode 3 include diffusion layers 22 and 32, respectively, to supply and diffuse fuel, catalyst layers 21 and 31, respectively, in which an oxidation/reduction reaction of fuel occurs, and electrode supporters 23 and 33, respectively.
A precious metal catalyst such as platinum that has excellent electrochemical properties even at low temperatures is used in catalyst layers 21 and 31. An alloy that includes a transition metal such as ruthenium, rhodium, osmium, nickel or the like is used to prevent catalyst poisoning that is caused by a carbon monoxide reaction by-product. Carbon paper, carbon cloth, or the like may be used for the electrode supporters 23 and 33. The carbon paper or carbon cloth is water-proofed so that fuel can be supplied easily and a reaction product can be discharged easily. The electrolyte membrane 1 is a polymer membrane that has a thickness of about 50 μm to about 200 μm and is a proton exchange membrane that contains moisture and has ion conductivity.
An electrode reaction in a direct methanol fuel cell (DMFC) that uses methanol and water as a mixed fuel includes an anode reaction in which fuel is oxidized and a cathode reaction caused by reduction of protons and oxygen. The anode, cathode, and overall reactions are shown below.    Anode reaction: CH3OH+H2O→CO2+6H++6e−    Cathode reaction: 3/2O2+6H++6e−→3H2O    Overall reaction: CH3OH+3/2O2→2H2O+CO2 
In the anode 2, carbon dioxide, protons, and electrons are generated by a reaction between methanol and water. The generated protons are transferred to the cathode 3 through the electrolyte membrane 1. In the cathode 3, water is generated by a reaction of protons, electrons that are transmitted via an external circuit (not shown), and oxygen. Thus, in a DMFC's overall reaction, methanol and oxygen react with each other and water and carbon dioxide are generated.
Theoretically, the voltage generated in a single DMFC is about 1.2 V and the open circuit voltage is equal to or less than about 1 V at room temperature under atmospheric pressure. The actual operating voltage is about 0.4 V to about 0.6 V because a voltage drop caused by an activation overpotential and a resistance overpotential occurs. Thus, in order to generate a desirably high voltage, a plurality of single cells should be connected in series.
A fuel cell stack is formed by stacking a plurality of single cells that are electrically connected in series. A bipolar plate 4, which is a conductive plate, is interposed between the single cells and couples the adjacent single cells together. A graphite block with excellent electrical conductivity, mechanical strength, and machining properties may be used as the bipolar plate 4. A block made of composite materials that contain a metal or a conductive polymer is also used as the bipolar plate 4.
Flow channels 41 and 42 that supply fuel (methanol) and air to the anode 2 and the cathode 3, respectively, are formed at both sides of the bipolar plate 4. An air channel 42 and a fuel channel 41 are formed at both sides of the bipolar plate 4 positioned in the middle of the stack. An end plate (not shown), which is a monopolar plate, supplies fuel or oxygen to each of the electrodes 2 and 3 and is disposed at ends of the stack. A channel (41 or 42 of FIG. 1) for supplying air or fuel to the contacting single cells is formed on the end plate.
FIG. 2 is a plan view of a surface of a conventional bipolar plate such as a surface on which a liquid fuel channel is formed. Referring to FIG. 2, in the conventional bipolar plate 4, a plurality of fuel/oxidant channels 41 are formed in a serpentine pattern in an electrode region 47 in which an MEA is disposed, so that upper portions of the fuel/oxidant channels 41 are opened. Manifolds 46 coupled with inlet and outlet of the fuel/oxidant channels 41, and fuel/oxidant path holes 43a, 43b, 44a, and 44b which communicate with the manifold 46 and through which liquid fuel or oxidant is supplied or discharged, are formed through the bipolar plate 4. The fuel/oxidant path holes 43a, 43b, 44a, and 44b form an inlet 43a and an outlet 43b of the liquid fuel and an inlet 44a and an outlet 44b of the oxidant.
In the flow channel with the serpentine shape shown in FIG. 2, the fuel/oxidant concentration gradient between the fuel/oxidant path hole 43a into which fuel/oxidant is flowed and the fuel/oxidant path hole 43b through which fuel/oxidant and a reaction product are discharged, is large. In addition, when the fuel/oxidant path holes 43a and 43b are formed at the same side, a plurality of flow paths may vary in length between the fuel/oxidant path holes 43a and 43b, and thus, flow velocities at the flow paths can be different. In addition, since the length of a flow path is large, a pressure loss is large.